Drum brake and brake shoe for one such brake

ABSTRACT

A drum brake comprising a plate on which at least one shoe having a friction lining is mounted. A lever moves the shoe against a wheel drum in response to a braking command to effect a brake application. A resilient return spaces the shoe apart from the drum when braking ceases. The shoe is mounted to slide along an essentially radial axis between two guide walls secured to the plate The lever for applying the shoe bears against the inside wall of the shoe for exerting a substantially radial force.

The invention relates to a drum brake including a plate on which atleast one shoe provided with a friction lining is mounted, applicationmeans for applying the shoe against a wheel drum in response to abraking command transmitted by a wheel cylinder, and resilient returnmeans for spacing the shoe apart from the drum when braking ceases.

BACKGROUND OF THE INVENTION

Conventional-type drum brakes include an abutment that is fixed or thatmoves relative to the plate to which a first end of the shoe is fixed,which first end is opposite from a second end in contact with the wheelcylinder, the wheel cylinder transmitting the braking command directlyto the shoe which is mounted to pivot relative to the plate at its endfixed to the abutment.

Drum brakes offer several advantages over disk brakes. For the sameapplication force, the braking torque is generally larger in a drumbrake than in a disk brake. The “handbrake” function is easier toinstall on a drum brake than on a disk brake. Therefore, drum brakes arestill used, in particular on the back wheels of vehicles.

However, replacing a drum brake shoe when its friction lining is wornrequires a relatively long disassembly and reassembly operation to beperformed. In addition, the stability of the braking torque could beimproved.

Because of the way forces exerted in the drum brake are distributedduring braking, the shoe is subjected to a roll-up phenomenon that canreduce the stability of the braking and therefore the stability of thevehicle.

During braking, four forces are remarkable in a drum brake: firstly aninlet force at the contact between the wheel cylinder and the first endof the shoe, secondly an application force at the contact between thelining and the drum, thirdly a braking force at the point of contactbetween the second end of the shoe and the abutment, and fourthly areaction force at the point of contact between the second end of theshoe and the abutment. The fact that the braking force and the reactionforce are applied at the same point tends to give rise to shoe roll-up.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is above all to provide a drum brake in whichshoe replacement is simplified, while braking torque stability isimproved.

In the invention, in a drum brake of the above-defined type the shoe ismounted to slide along an essentially radial axis between two guidewalls secured to the plate, and the application means for applying theshoe bear against the inside wall of the shoe of the drum, for exertinga substantially radial force.

In the invention, the braking force is separated from the applicationforce.

A shoe whose friction lining is worn can be replaced easily because theshoe can be removed merely by being moved in translation, since it isnot connected to the application means.

Preferably, the brake includes two shoes opposite from each other alonga diametral axis, and mounted to slide along an essentially radial axisbetween respective pairs of guide walls, and application means forapplying each shoe against the wheel drum in response to a brakingcommand, the application means bearing against the inside wall of theshoe.

The angle through which each shoe extends is less than 60°.

Each shoe comprises a rigid support, in particular a metal support, anda friction lining fixed to the outside surface of the support. The endsof the rigid support form abutments suitable for bearingcircumferentially against a corresponding guide wall.

The brake may be organized so that the axis of the reaction force isoffset relative to the axis of the inlet force.

The application means may comprise a lever that is flat, that is in theshape of an outwardly convex arc of a curve, and that has one of itsends hinged to a fixed point of the plate and its other end subjected toa thrust force, generally exerted by a wheel cylinder piston in adirection suitable for applying the shoe against the drum, the levercoming to bear via its radially outermost edge against the inside wallof the corresponding shoe.

At its outside edge, the lever may be provided with a projecting portionforming a hump that comes to bear against the inside wall of thecorresponding shoe. Preferably, the zone at which the lever bearsagainst the inside face of the shoe is situated about half-way betweenthe ends of the lever. The leverage ratio lies in the range 2 to 3.

The hinge whereby the lever is hinged to the plate may comprise, on theplate, a surface that is complementary with an end of the lever forguiding it.

The resilient return means may comprise first resilient return means forreturning the lever and second resilient return means for returning theshoe. The second resilient return means for returning the shoe maycomprise, at each circumferential end of the shoe, a spring clipengaging the plate and suitable for exerting a return force forreturning the shoe radially inwards.

The friction lining is bonded to the rigid support, advantageously bybeing overmolded thereon.

In a variant embodiment, the brake drum further comprises a lever whosecentral portion is provided with a hinge for a control bar in the shapeof an outwardly concave arc of a curve, each of the two ends of thecontrol bar bearing against the inside face of a respective shoe. Thedrum brake may have two levers, two control bars, and four shoes, theresulting set being symmetrical about a diameter of the plate.

The invention also provides a shoe for a drum brake as defined above,said shoe comprising a rigid support for friction lining, the peripheralends of said support forming abutments suitable for bearingcircumferentially against guide walls, for slidable mounting along anessentially radial axis, the friction lining being fixed against theoutside surface of the support.

The invention also provides a drum brake including a plate on which atleast one shoe provided with a friction lining is mounted, anapplication lever for applying the shoe against a wheel drum in responseto a braking command, and resilient return means for spacing the shoeapart from the drum when braking ceases, wherein, when the brakes areapplied, a reaction force is applied to the application lever and abraking force is applied to the shoe so that said braking force and saidreaction force are applied at two different points.

BRIEF DESCRIPTION OF THE DRAWINGS

Apart from the provisions set out above, the invention consists ofcertain other provisions that appear more explicitly below by means ofembodiments which are described in detail with reference to theaccompanying drawings, but which are in no way limiting.

In the drawings:

FIG. 1 a is a simplified elevation view, with portions seen from theoutside and with portions seen in section, showing a drum brake of aknown type;

FIG. 1 b is a simplified elevation view, with portions seen from theoutside and with portions seen in section, showing a drum brake of theinvention;

FIG. 2 is a perspective view of the brake as extracted from the drum;

FIG. 3 is a fragmented diagrammatic view of a variant embodiment;

FIG. 4 is a fragmentary diagrammatic view of a variant implementation ofhow the lever is hinged;

FIG. 5 is a fragmentary diagrammatic view of a variant implementation ofhow the lever bears against the shoe; and

FIGS. 6 to 8 are fragmentary diagrammatic views of other variantimplementations of how the lever bears against the shoe.

MORE DETAILED DESCRIPTION

FIG. 1 a shows a drum brake 202 of known type including a disk-shapedplate 204 organized to be fixed to a wheel arm of a vehicle, first andsecond shoes 210, 212 that are substantially circular arcuate in shapeand that are mounted on the plate, their convex portions 214 facingtowards the outside of the plate, and their convex portions 216 facingeach other, a wheel cylinder 215 secured to the plate 204 and disposedbetween respective first ends 218, 220 of the first and second shoes210, 212, and abutment means 222 secured to the plate 204 and disposedbetween second ends 224, 226 of the first and second shoes, and a drum(not shown) secured to the wheel and surrounding the convex portions 214of the shoes 210, 212 with clearance.

Since the first and second shoes 210, 212 are symmetrical, only thefirst shoe is described below, while also specifying what is specific toit relative to the second shoe 212.

The first shoe 210 is held laterally to the plate by a helical spring232 mounted in fixed manner relative to the plate 204 andperpendicularly thereto, and passing through the rim 228, said rim beingnipped between two turns of the spring 232.

The first shoe 210 is held laterally to the plate by a helical spring232 mounted in fixed manner relative to the plate 4 and perpendicularlythereto, and passing through the rim 228, said rim being nipped betweentwo turns of the spring 232.

The second ends 224, 226 of the first and second shoes are held inabutment against the abutment means 222 by coupling means 234 forcoupling together the two shoes, the coupling means 234 being formed inpart by a helical spring.

The drum brake 202 also includes return means 236 for urging the shoesback towards the inside of the plate 204 and formed in part by tworesilient elements, and a spacer 238 disposed below the wheel cylinderand parallel to the longitudinal axis X of the wheel cylinder. Thespacer is also provided with wear take-up means 240 for taking up wearin the brake linings, which means are of known type that is notdescribed in detail. The spacer 238 is held to the first and secondshoes 210, 212 by respective springs 241a and 241b of the return means236.

The wheel cylinder is fed in known manner via a hydraulic circuit, e.g.including a master cylinder and a power-assisted braking servomotoractuated by a brake pedal (none of these elements being shown). Thewheel cylinder has a first and second pistons (not shown) suitable formoving apart from each other along the axis X under drive from hydraulicfluid under pressure, the pistons then being applied against the firstends 218, 220 of the shoes 210, 212 and pushing them outwards.

The drum brake is also provided with a second actuator comprising aparking brake lever 242 that makes it possible to hold the motor vehiclestationary when it is parked, and that is not described in detail.

When the brakes are applied, with the drum turning in the directionindicated by arrow F, it can be observed that four forces are applied toeach shoe 210, 212. These four forces are described as applied to thefirst shoe 210.

An “inlet” first force Fe is applied at the contact between the wheelcylinder 215 and the first end 218 of the first shoe 210, an“application” second force Fa is applied at the contact between thelining 231 and the drum, a “braking” third force Ff is applied at thepoint of contact between the second end 224 of the shoe and the abutment222, and a “reaction” fourth force Fr is also applied at the point ofcontact between the second end 224 of the shoe and the abutment 222. Thefact that the braking force Ff and the reaction force Fr are applied atthe same point tends to give rise to shoe roll-up.

The present invention solves that problem.

FIG. 1 b shows a drum brake B including a plate 1 organized to be fixedto a vehicle wheel arm. Two shoes 2, 3 disposed opposite each otheralong a diametral axis X—X are mounted to slide along said axis betweenrespective guide walls 4 a & 4 b and 5 a & 5 b secured to the plate 1.

Each shoe 2, 3 is provided with a friction lining 2 a, 3 a fixed bybonding, preferably by overmolding, onto a rigid support 2 b, 3 b.

Each support 2 b, 3 b is formed by a metal strip. In the examples shownin FIGS. 1 and 2, the support strip 2 b, 3 b is curved to form acircular arc whose outside surface corresponds to a cylindrical surfaceportion that is convex facing radially outwards. The linings 2 a, 3 aare fixed to the cylindrical outside surfaces of the supports. Theperipheral ends of the supports 2 b, 3 b form abutments 6, 7 suitablefor pressing circumferentially against respective ones of the guidewalls 4 a to 5 b while retaining freedom of movement in radialtranslation. The abutments 6 and 7 can be constituted by the rectilinearedges of the supports 2 b, 3 b folded over parallel to the slide axisX—X.

The angle A through which each shoe 2, 3 extends is less than 60° andpreferably less than 50°, this numerical example being non-limiting. Theangle A is less than the angle through which a conventional drum brakeshoe extends, which makes it possible to reduce considerably the volumeof friction lining 2 a, 3 a, while retaining a substantially equivalentlife span. This reduction in lining volume compared with the liningvolume in a conventional drum brake can be as high as 60%.

FIG. 5 shows a variant in which the metal strip 2 b 1 is essentiallyplane, but with a piece in relief half way along its peripheral length,which piece in relief has generator lines perpendicular to the plane ofthe figure, forming a V-shaped projection G1 on the side opposite fromthe lining 2 a 1.

FIGS. 6 and 7 show metal strips 2 b 2, 2 b 3 that are plane. Thefriction lining in FIG. 7 is overmolded onto the strip 2 b 3.

FIG. 8 shows a metal strip 2 b 4 curved as in FIG. 1 b, but that,substantially half way along its dimension perpendicular to the plane ofthe figure, has a piece in relief forming a projection G4 with a planesurface on its side opposite from the friction lining 2 a 4, which isbonded to the strip 2 b 4.

The guide walls 4 a–5 b parallel to the diameter X—X are constituted byfaces of blocks that are secured to the plate 1 and that projectperpendicularly to the midplane of said plate.

The co-operation between the shoes 2 & 3 and the guide walls 4 a–5 b isorganized so as to enable the linings to be extracted or put in placebetween the guide walls by a movement in translation along an axisorthogonal to the midplane of the plate 1, i.e. orthogonal to the planeof FIG. 1 b.

The brake B is provided with application means 8, 9 for applying eachshoe 2, 3 against a wheel drum 10 that is shown in part, and that turnswith the wheel of the vehicle, e.g. in the direction indicated by arrowF in FIG. 1 b.

Each of the application means 8, 9 merely bears via a respective zone11, 12 against the inside wall of the corresponding shoe 2, 3 so as toexert a force that is substantially radial.

The application means 8 for applying the shoe 2 comprise a lever 13 thatis substantially in the shape of an arc of a curve, that is convexfacing outwards, and that has one end 13 a hinged about a fixed point ofthe plate 1 and its other end 13 b in abutment against a piston 14 of awheel cylinder 15 mounted on the plate. The piston 14 can exert a thrustforce on the end 13 b substantially parallel to the diameter X—X.

The lever 13 is flat, and advantageously cut out from a metal sheet. Onits radially outermost edge, and substantially half way between its ends13 a, 13 b, it has a projecting portion 16 forming a hump whoseperipheral edge constitutes the bearing zone 11.

The bearing zone 11, 12 of the lever 13, 19 via which it bears againstthe inside face of the shoe is situated about half-way between the endsof the lever. The leverage ratio is equal to the ratio between firstlythe distance between the hinge point 13 a and the point at which theforce from the piston 14 is applied to the lever and secondly thedistance between the hinge point 13 a and the bearing zone 11, 12. Saidleverage ratio advantageously lies in the range two to three.

In the example shown in FIGS. 1 and 6, the projection 16 or 16 a 2 has aconvex outside edge bearing against the support 2 b, 2 b 2. In FIG. 5,the outside edge of the projection 16 a 1 is rectilinear and bearsagainst the support 2 b 1. In FIGS. 1, 5, and 6, the configuration issuch that the axis of the inlet force and the axis of the reaction forcecoincide.

In FIGS. 7 and 8, the configuration is such that an offset exists, toone side or the other, between the axis X of the inlet force, and theaxis Z3, Z4 of the reaction force. In certain cases, a suitable offsetcan procure improved stability and improved lining wear; the performanceof the brake is improved. In FIG. 7, the projection 16 a 3 has a convexoutside edge bearing against the flat support 2 b 3. In FIG. 8, theprojection 16 a 4 bears via its convex outside edge against the planesurface of the projection G4.

In FIGS. 1 and 2, the end 13 a is circularly convex in part, and it isreceived in a cavity 17 constituting a complementary surface 13 a andprovided in a projection 18 of the plate 1. The end 13 a is also hingedto pivot about an axis that is perpendicular to the plate 1.

FIG. 4 shows a variant embodiment in which the end 13 a′ of the lever isconcave and the complementary surface 17′ integral with the plate isconvex, bearing against 13 a′.

The application means 9 comprise a lever 19 situated, relative to thelever 13, on the other side of an axis Y—Y orthogonal to X—X and passingthrough the center of the plate 1. The top end 19 b is pushed by anotherpiston 14 of the cylinder 15. The outside edge of the lever 19 bears viathe edge of a hump 20 against the shoe 3. The bottom end 19 a, which iscircular in part, is received in the circular cavity 21 in a projection22 of the plate 1. The way in which the hump 20 bears against the shoecan be as in variants shown in FIGS. 5 to 8.

A handbrake control lever 23 is hinged to a pin 24 carried by the lever19 in the vicinity of its end 19 b. That end 23 b of the lever which isopposite from the pin 24 is folded over so as to facilitate hooking atraction cable (not shown) that passes through a guide 25 fixed to theplate 1.

A spacer 26 is provided between the lever 13 and the levers 19 and 23 soas to limit the return stroke over which the ends 13 b, 19 b can returntowards each other under the action of first resilient return means E1(FIG. 2) tensioned between zones adjacent to said ends.

Second resilient return means E2 (FIG. 2) are provided for each shoe 2,3. At each circumferential end of the shoe 2, 3, each second resilientmeans E2 comprises a spring clip 27 engaging a projection 28 secured tothe plate 1 and orthogonal thereto. The clip 27 passes under theprojection 28. The guide walls such as 4 a are provided on the edge ofthe projection 28 facing the shoe 2 or 3. In its middle portion, eachguide wall is provided with a recess via which the clip 27 rises tohook, via teeth 29, onto the top edge of the support 2 b, 3 b and toreturn it radially inwards. The clips 27 make it possible to avoid or atleast to reduce rattle due to the shoes 2, 3 oscillating or vibrating.This example of resilient return means E2 is non-limiting, it beingpossible to use other equivalent means.

The spacer 26 provided between the lever 13 and the lever 23 and/or thelever 19 is preferably provided with a device 30 for automaticallytaking up wear in the friction linings by modifying the length of thespacer 26 in order to compensate for wear in the linings.

The levers 13, 19 can be held in the direction perpendicular to theplane of FIG. 1 b by means of springs 31 (FIG. 2), each having an axisperpendicular to the plate 1 and having one end connected to said plate.Each spring 31 is received in tight-fitting manner in a respective holein the lever, and enables the lever to move to a certain extent. Otherequivalent holding means can be provided for holding the levers.

The drum brake of FIGS. 1 and 2 operates as follows. It is assumed thatthe drum 10 turns in the direction indicated by arrow F of FIG. 1.

When the brakes are applied, liquid under pressure is sent into thecylinder 15 between the pistons 14 which move apart while pushing theends 13 b, 19 b of the levers 13 and 19. Said levers pivot in respectiveones of the cavities 17 and 21 and they exert substantially radialthrust via their humps 16, 20 against the support 2 b, 3 b of theassociated shoe. The shoe slides substantially radially between therespective guide walls, and comes to apply against the inside surface ofthe drum 10 for exerting the braking.

A friction reaction acting on the lining 2 a applies the downstream edgeof the support 2 b against the abutment wall 4 a in the directionindicated by arrow F. The shoe 2 is compressed against said abutment 4a. The shoe 3 is compressed against the abutment 5 b. The shoe thenapplies a braking force Ff against the guide wall 4 a in the directionindicated by arrow Ff.

A reaction force Fr then applies against the end 13 a of the shoe incontact with the abutment 18 as indicated by arrow Fr.

Therefore, the braking force Ff and the reaction force Fr apply at twodistinct points, thereby considerably increasing the stability of thebraking torque.

By leverage, the force applied by the piston 14 to the corresponding end13 b or 19 b is multiplied substantially by two at the hump 16, 20 andat the shoe 2, 3.

The two shoes 2, 3 working in compression makes it possible to reducethe volume of friction lining compared with the volume of frictionlining in a conventional drum brake, in which one of the shoes iscompressed and the other is pulled or tensioned. The forces are betterdistributed over the plate 1 and the stability of the braking torque isimproved.

Manufacturing is simplified compared with a conventional shoe in whichthe support 2 b, 3 b for the friction lining is bonded to thelever-forming portion 13, 19. In the invention, that bonding is omitted,with the wall 2 b, 3 b being free relative to the lever 13, 19 whichmerely bears against it.

The flat levers 13, 19 are made easy to assemble by means of the hinge 3a & 17 or 19 a & 22. The shoes 2, 3 are easy to replace when the linings2 a, 3 a are worn. After the drum has been removed to give access to theplate 1, the worn shoes can be extracted by moving them in translationalong an axis perpendicular to the plane of FIG. 1 b between the walls 4a & 4 b and 5 a & 5 b, and new shoes can be put back in place. Thelength of the adjustable spacer 26 is adjusted to take account of theincrease in thickness of the friction linings when they are replaced.

The handbrake can be actuated by exerting traction on the bottom end 23b of the lever 23 from right to left in FIG. 1 b, via a cable (notshown). The lever 23 turns clockwise as shown in FIG. 1 b about the pin24, and, via the spacer 26, pushes the lever 13 which applies the shoe 2against the drum 10. The lever 23 pivots against the spacer 26 byreaction, and pushes back the pin 24 and the lever 19 towards the rightof FIG. 1 b. This causes the shoe 3 to be applied against the drum 10.

FIG. 3 is a fragmentary diagrammatic view of a variant embodiment. Likeelements or elements that act similarly to the way in which the elementsdescribed above with reference to the preceding figures act aredesignated by like numerical references, possibly plus 100, and they arenot described again.

In its central portion, the lever 113 is provided with a hinge pin 32for a curved flat control bar 33 whose concave face faces outwards. Eachof the two ends 34, 35 of the control bar has a rounded convex shape,and bears against the inside face of a shoe 102 mounted to slideradially between walls 104 a, 104 b secured to the plate 101. The twoends 34, 35 of the control bar 33, and the associated shoes 102 aresituated on the same side of the diameter Y—Y of the plate 101 thatpasses through the middle of the cylinder 15.

When its top end 113 b is pushed by the piston 14, the lever 113controls the two associated shoes 102 via the control bar 33.

Another lever, another control bar, and two other shoes, symmetricalabout the axis Y—Y, are generally provided. The drum brake then has fourshoes 102 that are symmetrical in pairs about the axis Y—Y.

Regardless of the variant embodiment, the improvement in the stabilityof the braking torque can be reinforced by controlling and modulatingthe pressure of the fluid in the brake circuit.

Assembling the friction linings to the supports does not involveriveting or crimping.

Optionally, the bottom hinge points such as 13 a–17 of the levers can beomitted with another wheel cylinder similar to the cylinder 15 being putin place between the two bottom ends of the levers.

1. A drum brake including a plate on which a first brake having afriction lining and a second brake shoe having a friction lining aremounted, application means for applying a force moves said first andsecond brakes shoes against a wheel drum in response to a brakingcommand, and resilient return means for spacing said first and secondbrake shoes apart from the wheel drum when braking ceases, said firstand second brake shoes each being mounted to respectively slide along anessentially radial axis between first and second guide walls secured tosaid plate, said application means for moving said first and secondbrake shoes bearing against end exerting a substantially radial force onan inside wall of said first and second brake shoes through a leverarrangement including a first lever having a first end for receiving thebraking command and a second end that is located on said plate to definea first hinge and a second lever having a first end for receiving saidbraking command and a second end that is located on said plate to definea second hinge, a first control bar attached to said first lever througha first hinge pin and a second control bar attached to said second leverthrough a second hinge pin, said radial force being derived from thebraking command said first and second levers respectively pivoting aboutsaid first and second hinges and said first and second control barsrespectively pivoting about said first and second hinge pins such thatsaid first and second brake shoes move along a radial plane with respectto said first and second guide walls.
 2. The drum brake according toclaim 1, wherein a leverage ratio of the braking command and the radialforce lies in the range 2 to
 3. 3. The drum brake as recited in claim 2wherein said first brake shoe has a first segment and a second segmentthat are spaced apart from each other and said first control bar has afirst end that engages an inside face on said first segment thereof anda second end that engages an inside face on said second segment thereofand where said second brake shoe has a first segment and a secondsegment that are spaced apart from each other and said second controlbar has a first end that engages an inside face on said first segment ofsaid second brake shoe and a second end that engages an inside face onsaid second segment of said second brake shoe such that the radial forcederived from the braking force is uniformily applied through the firstand second brake shoes to effect a brake application.
 4. The drum brakeaccording to claim 3, wherein the drum brake is organized so that anaxis of the reaction force is offset relative to an axis of the inletforce.
 5. The drum brake as recited in claim 3 wherein a zone at whichthe first and second control bars respectively bear against the insideface of the first and second segments of said first and second shoes issituated about half-way between the ends of each segment.